Accelerating: Radiofrequency cavities

To accelerate particles, the accelerators are fitted with metallic chambers containing an electromagnetic field known as radiofrequency (RF) cavities. Charged particles injected into this field receive an electrical impulse that accelerates them.

Assembly of cavities for the HIE-ISOLDE accelerator (Image: Maximilien Brice/CERN)

In the Large Hadron Collider (LHC), 16 RF cavities are housed in four cylindrical refrigerators called cryomodules, which enable them to work in a superconducting state.

Each cavity is driven by a high-power klystron, which is a tube containing electron beams. The electron beams are intensity-modulated to a frequency of 400 MHz, or 400 million oscillations per second. A rectangular pipe of conducting metal called a waveguide directs energy to the cavity. The cavity’s shape has been specifically designed to achieve resonance and the build up in intensity of the electromagnetic waves. Each cavity can reach a maximum voltage of 2 megavolts (MV), corresponding to 16 MV per beam.

One of the module containing the accelerating cavities for the LHC (Image: Maximilien Brice/CERN)

The LHC’s RF cavities bring the 450 GeV energy of the particles (1 GeV = 1 billion electronvolts) to 6.5 TeV (1 TeV = 1 million million electronvolts) - more than 14 times their injection energy. The maximum energy is reached in around 20 minutes with the bunches having passed through the RF cavities more than 10 million times.

The field in an RF cavity is made to oscillate (switch direction) at a given frequency, so timing the arrival of particles is important. In the LHC, each RF cavity is tuned to oscillate at 400 MHz. When the beam has reached the required energy, an ideally timed proton with exactly the right energy will not be accelerated. By contrast, protons with slightly different energies arriving earlier or later will be accelerated or decelerated so that they stay close to the desired energy. In this way, the particle beam is sorted into packs of protons called "bunches".

Assembly of two crab cavities for the High-Luminosity LHC (Image: Julien Ordan/CERN)

Aside from these accelerating cavities, CERN is developing “crab” cavities for the LHC’s successor, the High-Luminosity LHC. The purpose of these crab cavities is to give a transverse momentum to steer the particles as they approach the collision point.